The o-rings on my dob perished and fell off - exact size (measured) replacements have been sourced, but do you fit them at A (in the groove) or B (above the flange) - see photo.
They had fallen off so I could not check the original position - nothing in the manual either. In the groove (A) seems to do nothing, above (B) is preventing the spring clips from engaging.
Maybe its just a matter of them needing to flex a bit with use?